From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-20.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3FEABC43461 for ; Fri, 9 Apr 2021 12:05:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 238E1610A4 for ; Fri, 9 Apr 2021 12:05:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233689AbhDIMFg (ORCPT ); Fri, 9 Apr 2021 08:05:36 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53222 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232469AbhDIMF0 (ORCPT ); Fri, 9 Apr 2021 08:05:26 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 09D19C061762; Fri, 9 Apr 2021 05:05:13 -0700 (PDT) Date: Fri, 09 Apr 2021 12:05:10 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1617969911; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ON0hoCYdzUbTivaYwjn2vHtjQiI9qVDaizymTlQ3iNc=; b=gqXnwQR996X9aXxcLzPiuMdf9d0/40WyLzW3bsCDcRJM/tCO53k8HsIHrXAeooL4NPeVSE bZlXP9HkGKtJdI1GwKg//IW5G7szHsApeLY4EWJJEByHsr+1YV7/vksfM/Wg465uFfhIfl tzzyIFxdd/5XHo3DrFl+x40XTsq3yZvyWkzmBKKFxSBR5bijXieOS5FafKR+6diU7jJd9+ HX2li3QNzYkhIqkfmT9jNmiN6Bnkt4/NTbYZw6Bq++cRM/J0+ZOiynOpm4rG7zsHbCS14y fcgI7A9MnhJYEyjEE6j0HMh4QzN83UnfrEHK3pJ4FwG/3H4z1POuY/th8ZmwcQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1617969911; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ON0hoCYdzUbTivaYwjn2vHtjQiI9qVDaizymTlQ3iNc=; b=2tpmZnbmBiykhHwPh85cAKTdUOiWnaH4Av0+IFsiwN7IO45xGa76FZmuasKlO4V7hifDzl 2XMA/99uQ/2AL9AA== From: "tip-bot2 for Lingutla Chandrasekhar" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: sched/core] sched/fair: Ignore percpu threads for imbalance pulls Cc: Lingutla Chandrasekhar , Valentin Schneider , "Peter Zijlstra (Intel)" , Dietmar Eggemann , Vincent Guittot , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20210407220628.3798191-2-valentin.schneider@arm.com> References: <20210407220628.3798191-2-valentin.schneider@arm.com> MIME-Version: 1.0 Message-ID: <161796991092.29796.13468768733654328424.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the sched/core branch of tip: Commit-ID: 8d25d10a4f5a5d87c062838358ab5b3ed7eaa131 Gitweb: https://git.kernel.org/tip/8d25d10a4f5a5d87c062838358ab5b3ed7eaa131 Author: Lingutla Chandrasekhar AuthorDate: Wed, 07 Apr 2021 23:06:26 +01:00 Committer: Peter Zijlstra CommitterDate: Fri, 09 Apr 2021 13:52:10 +02:00 sched/fair: Ignore percpu threads for imbalance pulls During load balance, LBF_SOME_PINNED will be set if any candidate task cannot be detached due to CPU affinity constraints. This can result in setting env->sd->parent->sgc->group_imbalance, which can lead to a group being classified as group_imbalanced (rather than any of the other, lower group_type) when balancing at a higher level. In workloads involving a single task per CPU, LBF_SOME_PINNED can often be set due to per-CPU kthreads being the only other runnable tasks on any given rq. This results in changing the group classification during load-balance at higher levels when in reality there is nothing that can be done for this affinity constraint: per-CPU kthreads, as the name implies, don't get to move around (modulo hotplug shenanigans). It's not as clear for userspace tasks - a task could be in an N-CPU cpuset with N-1 offline CPUs, making it an "accidental" per-CPU task rather than an intended one. KTHREAD_IS_PER_CPU gives us an indisputable signal which we can leverage here to not set LBF_SOME_PINNED. Note that the aforementioned classification to group_imbalance (when nothing can be done) is especially problematic on big.LITTLE systems, which have a topology the likes of: DIE [ ] MC [ ][ ] 0 1 2 3 L L B B arch_scale_cpu_capacity(L) < arch_scale_cpu_capacity(B) Here, setting LBF_SOME_PINNED due to a per-CPU kthread when balancing at MC level on CPUs [0-1] will subsequently prevent CPUs [2-3] from classifying the [0-1] group as group_misfit_task when balancing at DIE level. Thus, if CPUs [0-1] are running CPU-bound (misfit) tasks, ill-timed per-CPU kthreads can significantly delay the upgmigration of said misfit tasks. Systems relying on ASYM_PACKING are likely to face similar issues. Signed-off-by: Lingutla Chandrasekhar [Use kthread_is_per_cpu() rather than p->nr_cpus_allowed] [Reword changelog] Signed-off-by: Valentin Schneider Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Dietmar Eggemann Reviewed-by: Vincent Guittot Link: https://lkml.kernel.org/r/20210407220628.3798191-2-valentin.schneider@arm.com --- kernel/sched/fair.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d0bd861..d10e33d 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7598,6 +7598,10 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env) if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu)) return 0; + /* Disregard pcpu kthreads; they are where they need to be. */ + if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p)) + return 0; + if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) { int cpu;